1. Technical Field
The present invention is related to the field of optical amplifiers and, more particularly, to a method and apparatus for determining the absolute noise figure of optical amplifiers.
2. Description of the Prior Art
For many communication applications utilizing optical amplifiers, it is important to know the noise figure of the optical amplifier itself. In particular, the amplifier noise figure is an important factor in determining the input power required to maintain a given bit error rate (BER). In effect, the noise figure is a measure of the amplifier's sensitivity and is a useful parameter to determine when the amplifier is used as a pre-amp in an optical receiver.
An article entitled "Measurement of Semiconductor Laser Amplifier Noise Figure in Coherent Optical Transmission System", by G. R. Walker et al., appearing in Electronics Letters, Vol. 25, No. 25, December 1989 at pp. 1681-2 describes two different techniques for determining the noise figure of semiconductor laser optical amplifiers. The first technique utilizes the direct measurement of the ratio of the amplifier gain to the spontaneous emission spectral density. Various other broadband noise sources, such as the noise floor of the analyzer itself, local oscillator shot noise, spontaneous shot noise, spontaneous-spontaneous beat noise, and local oscillator-spontaneous beat noise, were cited by the authors as being superimposed on the desired quantity, with only the local oscillator-spontaneous beat portion capable of being separately calculated and removed. The alternative determination technique related to the operation of a semiconductor amplifier in a 565 Mbit/s heterodyne transmission system. Again assuming that various other noise source contributions would be minimal, the amplifier noise figure was determined by evaluating the input/output signal power ratio.
An obvious limitation on both of these approaches is the assumption regarding the relative contribution of the various other noise sources, and the resultant error in the calculated noise figure from the presence of the other noise sources.
Therefore, a need remains in the art for a method of determining the noise figure of an optical amplifier which provides a result that is essentially independent of the various other noise sources within the system.